A lithographic apparatus is a machine configured to apply a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs) and other devices. In that instance, a patterning device (e.g., a mask or a reticle) may be used to generate a circuit pattern to be formed on an individual layer of the IC. In such a case, a patterning device (e.g., a mask) may contain or provide a pattern corresponding to an individual layer of the device (“design layout”), and this pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g., silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (“resist”) provided on the substrate, by methods such as irradiating the target portion through the pattern on the patterning device. In general, a single substrate contains a plurality of adjacent target portions to which the pattern is transferred successively by the lithographic apparatus, one target portion at a time. In one type of lithographic apparatuses, the pattern is transferred onto one target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus, commonly referred to as a step-and-scan apparatus, a projection beam scans over the patterning device in a given reference direction (the “scanning” direction) while synchronously moving the substrate parallel or anti-parallel to this reference direction. Different portions of the pattern on the patterning device are transferred to one target portion progressively. Since, in general, the lithographic apparatus will have a magnification factor M (generally <1), the speed F at which the substrate is moved will be a factor M times that at which the beam scans the patterning device.
Prior to transferring the pattern from the patterning device to the substrate, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the transferred pattern. This array of procedures is used as a basis to make an individual layer of a device, e.g., an IC. The substrate may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off the individual layer of the device. If several layers are required in the device, then the whole procedure, or a variant thereof, is repeated for each layer. Eventually, a device will be present in each target portion on the substrate. These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc.
As noted, lithography is a central step in the manufacturing of ICs and other devices, where patterns formed on substrates define functional elements of the devices, such as microprocessors, memory chips etc. Similar lithographic techniques are also used in the formation of flat panel displays, micro-electro mechanical systems (MEMS) and other devices.
In a lithographic process (i.e., a process of developing a device or other structure involving lithographic exposure, which may typically include one or more associated processing steps such as development of resist, etching, etc.), it is desirable frequently to make measurements, e.g., for process control and verification. Various tools for making such measurements are known, including alignment sensor systems to measure alignment between structures, topography measurement systems (focus/leveling systems), scanning electron microscopes, which are often used to measure critical dimension (CD), and/or specialized tools using marks to measure various parameters, such as overlay (i.e., the accuracy of alignment of two layers of a substrate), focus used to expose a pattern, a dose used to expose a pattern, CD, etc.